Shot peening is a type of cold working in which shot media that are iron or non-ferrous metal spheres are hit against a metal surface at a high speed, thereby generating a compressive stress on the metal surface and improving fatigue strength against cyclic load. In a pressure vessel in a chemical plant, a reactor vessel or the like, shot peening is performed on, for example, a welded part so as to improve the fatigue strength of a welding joint part.
Conventionally, for example, Patent Literature 1 discloses an ultrasonic shot peening apparatus that performs shot peening on a J-welded part between a lower surface of a reactor vessel cover and a nozzle stub and on a surface adjacent to the J-welded part. This ultrasonic shot peening apparatus includes a disk, ultrasonic peening heads each having an oscillator formed on the disk and inserted into collection holes, and a chamber that is arranged in a state of surrounding an outer circumferential surface of the disk while contacting the outer circumferential surface, that has a cylindrical shape, and an upper edge of which is inclined similarly to a shot-peening worked portion.